As btrfs showed, a partial read of data in AIO /O_DIRECT circumstances can
really confuse MariaDB.
Filipe Manana (SuSE)[1] showed how database programmers can assume
O_DIRECT is all or nothing.
While a fix was done in the kernel side, we can do better in our code by
requesting that the rest of the block be read/written synchronously if
we do only get a partial read/write.
Per the APIs, a partial read/write can occur before an error, so
reattempting the request will leave the caller with a concrete error to
handle.
[1] https://lore.kernel.org/linux-btrfs/CABVffENfbsC6HjGbskRZGR2NvxbnQi17gAuW65eOM+QRzsr8Bg@mail.gmail.com/T/#mb2738e675e48e0e0778a2e8d1537dec5ec0d3d3a
Also spell synchronously correctly in other files.
Fixed tpool::pread() and tpool::pwrite() to return SSIZE_T on Windows,
so that huge numbers are not converted to negatives.
Also, make sure to never attempt reading/writing more bytes than
DWORD can accomodate (4G)
Do not execute user callback just after pwrite. Instead, submit user
function as task into thread pool. This way, the IO thread would not hog
aiocb, which is a limited (in Innodb) resource
aio_linux::m_max_io_count: Unused data member; remove.
aiocb::m_ret_len: Declare as the more compatible type size_t.
Unfortunately, ssize_t is not available on Microsoft Visual Studio.
The library is capable of
- asynchronous execution of tasks (and optionally waiting for them)
- asynchronous file IO
This is implemented using libaio on Linux and completion ports on
Windows. Elsewhere, async io is "simulated", which means worker threads
are performing synchronous IO.
- timers, scheduling work asynchronously in some point of the future.
Also periodic timers are implemented.
